I just had a new 355 put together in neutral/park i can rev it all i want with no problem but when i drive it at about 70mph it starts to rattle if igive it more gas the rattle stops,i have added octane booster,checked the timing ect.all the valve adjustments were done by a speed shop.I did find a steady oil leak at the rear of the intak manifold could that be causing a oil pressure loss/lifter rattle? suggestions please i have been trying to figure this out all day.

 

Have you checked to see if everything is tight? Does the rattle get worse as RPM's increase?